1. Field of the Invention
The present invention relates to a method of depolarizing a ferroelectric material using electrical means.
As regards this invention the term depolarization will be used to indicate a complete depolarization or a partial depolarization of the said material.
The ferroelectric materials which can be used are for example PVDF (polyvinylidene fluoride), terpolymers or certain copolymers like VF.sub.2 /VF.sub.3. In a more general way these materials are crystals, polycrystals, polymers or copolymers.
The invention also relates to a procedure to depolarize certain zones of a ferroelectric material selectively, as for example zones of a film or a foil of such a material.
Advantageously, this procedure applies to the production of materials used for different kinds of sensors: piezoelectric pressure gages, pyroelectric transducer etc. In particular, it will be possible to adopt various configurations, as for example a matrix configuration. This configuration can be obtained by depolarizing the whole surface of a foil of ferroelectric material which had been polarized before with the exception of those zones which are regularly distributed according to said matrix configuration.
Finally the invention relates to a procedure which can be used to obtain ferroelectric materials of reinforced dielectric strength.
2. Description of Related Art
It is of course necessary to polarize a ferroelectric material before it can be depolarized completely or partially. Various polarization methods are known.
The applicant has described a method and a facility to polarize ferroelectric materials in the French patent application FR-A-2 538 157. The findings of this invention can well be used in the context of the present invention. According to the described method an alternating electric field of low frequency is applied to the ferroelectric material, and then the amplitude of this field is progressively increased in such a way that there is a controlled residual polarization left in the material
It is also possible to determine the hysteresis loop: polarization P as a function of the electric field E.
The enclosed FIG. 1 illustrates such a hysteresis loop which is also shown, for example, in FIG. 4B of the above-mentioned patent application.
The polarization of the material can theoretically be set to zero by applying a field E.sub.1 =E.sub.c where E is the coercive field strength. The polarization can as well be decreased without setting it to zero if a field E.sub.2 is applied where E.sub.2 &gt;E.sub.c. These values are of course positive or negative in conformity with the initial polarization.
However, this method is not very precise. In fact, the coercive field strength E.sub.c and the hysteresis loop are dependent on the thickness of the material. This thickness is not uniform and the field strength E.sub.c varies as a function of the field. In addition, a ferroelectric material as for example PVDF or VRF.sub.2 is never homogeneous. As a consequence the achieved depolarization can never be complete or it is not complete in a reproducible way.
In addition, the induced electric field is anisotropic in the material which is of a semicrystalline type in the example considered.
If a specific partial depolarization shall be achieved, it will be even more difficult to determine the value of the field E.sub.2 to be applied. This is in particular due to the slope of the hysteresis loop.
The invention aims at reducing the disadvantages of the existing methods which have just been described.